1. Field of the Invention
The present invention relates to an organic photoreceptor used in the field of a copying machine and printer, a manufacturing method of such an organic photoreceptor, and a process cartridge an image forming apparatus using this organic photoreceptor.
2. Related Art
In recent years, an organic photoreceptor (hereinafter also referred to as “photoreceptor”) has been utilized over an extensive range in the electrophotographic photoreceptor. The organic photoreceptor has advantages over other types of photoreceptor, such as easier development of materials conforming to various types of light sources for exposure ranging from visible light to infrared light, possible selection of materials free of environmental pollution and lower production costs. However, the organic photoreceptor is characterized by poorer mechanical strength, easier deposition of foreign substances, poorer chemical resistance, earlier deterioration of electrostatic characteristics at the time of printing multiple sheets, and susceptibility to scratches on the surface.
To be more specific, an organic photoreceptor is required to have a sufficient resistance (wear resistance) to adhesion of foreign substances on the surface or damages caused by the external mechanical force applied when a toner image formed on the photoreceptor is transferred onto a transfer material such as paper or when toner remaining on the photoreceptor is removed by cleaning.
A method of impregnating the extreme surface of the photoreceptor with a fluorine-containing resin such as polytetrafluoroethylene (PTFE) is known in the prior art to improve the resistance of the photoreceptor to adhesion of foreign substances on the surface or damages. In particular, the art of using fine particles of fluorine-containing resin having a small crystallinity (the half-width of the X-ray diffraction peak: 0.28 or more) and a small diameter has been reported (Official Gazette of Japanese Patent Tokkaihei 8-328287 (claim 1 on page 2)) as a method of effectively reducing the friction coefficient on the surface of the photoreceptor to improve the wear resistance.
However, reduction in crystallinity will cause an increase in the spreading property of the fine particles of fluorine-containing resin and a decrease in dispersion stability of fine particles of fluorine-containing resin in the coating dispersion. This results in coagulation among particles, accompanied by difficulties in forming a membrane having uniform characteristics. Thus, black spots (strawberry-formed spotted image), black streaks or white streaks are formed on the electrophotographic image, with the result that a serious deterioration of the image occurs. Such problems have occurred in the prior art. In particular, in fine particles of fluorine-containing resin having a small crystallinity and an average particle diameter of less than 0.20 μm, there was a considerable reduction in the stability of coating dispersion of the fine particles of fluorine-containing resin. When film formation was made by the dip coating method for keeping this dispersion retained for a long time, it was difficult to form a surface layer having uniform surface energy because of the coagulum occurring in the dispersion, with the result that the aforementioned black spots, black streaks or white streaks were found out.
The art of using the fine particles of fluorine-containing resin having a small diameter is known as a method for effective reduction of a friction coefficient on the surface of the photoreceptor. Use of the fine particles having a small diameter is effective since it increases the surface area for the same number of parts to be added. However, as described above, when the diameter of fine particles of fluorine-containing resin having a small crystallinity was reduced, the uniformity in dispersion further deteriorated. It became difficult to form a uniform and smooth film free of coagula. Remarkable deterioration of the image quality was observed from the initial phase of printing.